arXiv subjects

A. Bik

Publications and source records attributed to A. Bik.

61 records · Page 4Linked to original sources

arXiv2005

Detection of steam in the circumstellar disk around a massive Young Stellar Object

We report on the observation of hot water vapor (steam) in the inner AU of a young massive star located in the star-forming region IRAS 08576-4334. The water lines are detected in a medium resolution (R=10,000) K-band spectrum taken by the infrared spectrometer ISAAC mounted on the VLT-ANTU. The water vapor is at a mean temperature of 1565+/-510 K, cooler than the hot CO found in the same object, which is at 1660K and the column density is N(H2O)=(2.5 +/- 0.4)x1E18 cm-2. The profile of both H2O and CO lines is best reproduced by the emission from a Keplerian disk. To interpret the data, we also investigate the formation of molecules and especially CO and water vapor in the inner hot and dense part of disks around young high mass stars using a pseudo time-dependent gas-phase chemical model. Molecules are rapidly photodissociated but this destruction is compensated by an efficient formation due to fast neutral-neutral reactions. The ability of CO molecules to self-shield significantly enhances its abundance. Water molecules are sufficiently abundant to be detectable. The observed H2O/CO ratio is reproduced by gas at 1600K and an enhanced UV field over gas density ratio I_UV/nH=1E(-4)- 1E(-6). The simulations support the presence of CO and H2O molecules in the inner disks around young massive stars despite the strong UV radiation and show that the OH radical plays an essential role in hot gas chemistry.

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arXiv2004

Evidence for a hot dust-free inner disk around 51 Oph

We report on the observation of CO bandhead emission around 51 Oph (dv=2). A high resolving power (R~10,000) spectrum was obtained with the infrared spectrometer ISAAC mounted on VLT-ANTU. Modeling of the profile suggests that the hot (Tgas=2000-4000K) and dense (n_H>1e10 cm-3) molecular material as probed by the CO bandhead is located in the inner AU of a Keplerian disk viewed almost edge-on. Combined with the observation of cooler gas (Tgas=500-900K) by ISO-SWS and the lack of cold material, our data suggest that the disk around 51 Oph is essentially warm and small. We demonstrate the presence of a dust-free inner disk that extents from the inner truncation radius until the dust sublimation radius. The disk around 51 Oph may be in a rare transition state toward a small debris disk object.

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arXiv2004

Evidence for an inner molecular disk around massive Young Stellar Objects

We present observations of CO overtone bandhead emission toward four massive Young Stellar Objects (spectral type O6--B5). The high signal-to-noise ratio K-band spectra were obtained with VLT-ISAAC at a resolution of 30 km/s, sufficient to resolve the bandheads, but not the individual J-lines. We are able to explain the shape of the lines by assuming a simple isothermal keplerian disk model seen at different inclinations. The gas temperature ranges from 1500 to 4500 K and the CO column density is between 0.1 and 4 10^21 cm^-2. The emission probably arises within the first few astronomical units of the disk, consistent with the high gas temperature. Our results indicate that molecules can survive close to a hot star and suggest that dense (n_H > 10^10 cm^-3) inner disks may be relatively common at an advanced stage of high-mass star formation.

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arXiv2003

The peculiar circumstellar environment of NGC2024-IRS2

We re-examine the nature of NGC2024-IRS2 in light of the recent discovery of the late O-type star, IRS2b, located 5 arcsec from IRS2. Using L-band spectroscopy, we set a lower limit of Av = 27.0 mag on the visual extinction towards IRS2. Arguments based on the nature of the circumstellar material, favor an Av of 31.5 mag. IRS2 is associated with the UCHII region G206.543-16.347 and the infrared source IRAS 05393-0156. We show that much of the mid-infrared emission towards IRS2, as well as the far infrared emission peaking at ~ 100 micron, do not originate in the direct surroundings of IRS2, but instead from an extended molecular cloud. Using new K-, L- and L'-band spectroscopy and a comprehensive set of infrared and radio continuum measurements from the literature, we apply diagnostics based on the radio slope, the strength of the infrared hydrogen recombination lines, and the presence of CO band-heads to constrain the nature and spatial distribution of the circumstellar material of IRS2. Using simple gaseous and/or dust models of prescribed geometry, we find strong indications that the infrared flux originating in the circumstellar material of IRS2 is dominated by emission from a dense gaseous disk with a radius of about 0.6 AU. At radio wavelengths the flux density distribution is best described by a stellar wind recombining at a radius of about 100 AU. Although NGC2024/IRS2 shares many similarities with BN-like objects, we do not find evidence for the presence of a dust shell surrounding this object. Therefore, IRS2 is likely more evolved.

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arXiv2003

A VLT spectroscopic study of the ultracompact HII region G29.96-0.02

A high quality, medium-resolution K-band spectrum has been obtained of the ultracompact HII region G29.96-0.02 with the Very Large Telescope (VLT). The slit was positioned along the symmetry axis of the cometary shaped nebula. Besides the spectrum of the embedded ionizing O star, the long-slit observation revealed the rich emission-line spectrum produced by the ionized nebula with sub-arcsec spatial resolution. The nebular spectrum includes Br$γ$, several helium emission lines and a molecular hydrogen line. A detailed analysis is presented of the variation in strength, velocity and width of the nebular emission lines along the slit. The results are consistent with previous observations, but the much better spatial resolution allows a critical evaluation of models explaining the cometary shape of the nebula. Our observations support neither the wind bow shock model nor the champagne flow model. The measured line ratios of the nebular hydrogen and helium lines are compared to predictions from case B recombination-line theory. The results indicate an electron temperature between 6400 and 7500 K, in good agreement with other determinations and the Galactocentric distance of 4.6 kpc. The He$^{+}$/H$^{+}$ ratio is practically constant over the slit; we argue that He is singly ionized throughout the nebula. We review the various observational constraints on the effective temperature of the ionizing star and show that these are in agreement with its K-band spectral type of O5-O6V.

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arXiv2003

Identification of the ionizing source of NGC 2024

We propose the late-O, early-B star IRS2b as the ionizing source of the Flame Nebula (NGC 2024). It has been clear that such a hot, massive star must be present in this heavily obscured region, and now it has been identified. New near-infrared photometry shows that IRS2b is the most luminous and hottest star in the young star cluster embedded in the center of NGC 2024. The near-infrared observations (5' x 5') cover ~90 % of the HII region detected in radio continuum radiation, making the probability very low that the ionizing star is not present in the field. A K-band spectrum of IRS2b obtained with ISAAC on the Very Large Telescope indicates that the spectral type of IRS2b is in the range O8V - B2V. Additional arguments indicate that its spectral type is likely closer to O8 than to B2. The corresponding amount of ionizing radiation is consistent with published radio continuum and recombination line observations.

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arXiv2002

Clusters in the inner spiral arms of M51: the cluster IMF and the formation history

We study the cluster population in a region of 3.2x3.2 kpc^2 in the inner spiral arms of the intergacting galaxy M51, at a distance of about 1 to 3 kpc from the nucleus, based on HST--WFPC2 images taken through five broadband and two narrowband filters. We found 877 cluster candidates and we derived their ages, initial masses and extinctions by comparing their energy distribution with the Starburst99 cluster models. We describe the 3 and 2-dimensional least-square energy fitting method that was used (3DEF, 2DEF). The lack of [OIII] emission in even the youngest clusters with strong H-alpha emission, indicates the absence of the most massive stars and suggests a mass upper limit of about 25 to 30 solar masses. The mass versus age distribution of the clusters shows a drastic decrease in the number of clusters with age, which indicates that cluster disruption is occurring on a timescale of about 10 Myr for low mass clusters. The cluster initial mass function for clusters younger than 10 Myr has an exponent of alpha = 2.0 (+- 0.05) We derived the cluster formation history from clusters with an initial mass larger than 10^4 solar masses. There is no evidence for a peak in the cluster formation rate within a factor two at about 200 to 400 Myr ago, i.e. at the time of the interaction with the companion galaxy NGC 5194.

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